1. Field of the Invention
The present invention relates to a process for making scrubber sludge stable so that it can be safely disposed. More specifically, the process relates to fusing or combining together the many small particles of gypsum and flyash in the scrubber sludge by injecting them into a boiler and heating them sufficiently to soften or melt their surfaces and impinging them on each other, or even to melting the small particles together and having the resulting larger agglomerates fall out the bottom of the boiler.
2. Description of the Prior Art
In the production of electricity by steam, very often coala is burned to supply the heat to raise the steam. Coal contains sulfur; some coal contains a little sulfur and some a lot, but all coal contains sulfur. As the coal is burned the sulfur is burned to sulfur dioxide (SO.sub.2). The SO.sub.2 is a gas and it goes out of the stack with the other products of combustion. Some sulfur may be discharged from the mills as pyrite and a small amount can be retained in the ash, but most of the sulfur in the coal exits the boiler as the gas, SO.sub.2. This gas is an air pollutant, is not healthy to breathe, contributes to smog, and is oxidized in the atmosphere to sulfur trioxide which combines with water to form the corrosive and acidic component of acid rain, sulfuric acid. As a result there are numerous local, state and federal laws and regulations limiting the emissions of sulfur dioxide. A response to such regulations which is often followed in large electric power plants is to install sulfur scrubbers.
Typically these scrubbers contact a slurry of lime or limestone with the flue gas from the combustion process. Usually the by-product is gypsum, CaSO.sub.4.2H.sub.2 O, slurried in water and mixed with the flyash which is typically removed from the gas by the slurry in the scrubber. Thus the by-product of the scrubber is a sludge containing large amounts of water, gypsum, and flyash. The product, of course, is clean flue gas. Various efforts have been made to convert the sludge into useful plaster of Paris, wall board, or other useful products, and some have had limited success. However, the great bulk of the sludge must be disposed. The sludge is often disposed near the power plant in ponds or impoundments, but on occasion it may be transported some distance and placed in landfills.
The sludge contains mineral matter which has various solubilities in water. Some toxic metals are among that mineral matter. However, the United States Environmental Protection Agency has determined that the state of being hazardous depends upon extraction rates of the toxic metals. This in turn depends, among other things, on particle size. Unfortunately the flyash that is collected in the sludge may have a mass mean particle diameter as low as 20 micrometers. These very small particles have a large surface area to volume ratio and can be expected to be more easily leached than larger particles.
Bottom ash, due to its larger size, will be less of a leaching hazard. The United States Environmental Protection Agency has established extraction tests to determine if coal ash is hazardous. The present procedures are set forth in 40 CFR 260.20 and 260.21. It is emphasized that the test of ash for being hazardous is based on how much of a given element is extractable from a sample, not on how much is in a sample. The sample is crushed to pass a 3/8-inch (9.5 mm) sieve and extracted with water to which acetic acid is added to keep the pH at 5.0. The sample is contacted with the weak acid for 24 hours, after which time the liquid is tested for metals. The extract is tested for arsenic, barium, cadmium, chromium, lead, mercury, selenium, and silver. A concentration limit is specified for each metal and if one exceeds the specified limit the ash is considered as having EP Toxicity and considered a hazardous waste. It is well known that disposal of hazardous waste is very expensive and should be avoided if possible.
It is true and recognized by people familiar with the arts of extraction and lixiviation that soluble materials are much more readily extracted from small particles than from large particles. Because small particles have higher surface area/volume ratios than large particles, a higher proportion of the soluble materials are at the surface of the particle and come into contact with the extraction liquid. Therefore, ash with large particles will often be judged non-toxic, while the same ash having small particle sizes would be found to be toxic. Thus, by increasing the size of ash particles they can be made more safe for disposal. Because of their small size the sample crushing procedure specified in the test is not relevant to flyash particles. It would take over 100 million spheres of flyash which on average is 20 micrometers in diameter to make one, 9.5 millimeter diameter sphere.